1. Field of the Invention
The invention relates to a method providing improved bi-layer photoresist patterning.
2. Description of the Related Art
There is a desire in the industry to achieve higher circuit density. On method of achieving higher density is to provide improved resolution of circuit patterns in resist fields. On technique for doing this is by using bi-layer photoresist methods. Bi-layer photoresist methods are described in U.S. Pat. No. 5,985,524, issued Nov. 16, 1999 to Allen et al., entitled, “Process For Using Bilayer Photoresist;” and in U.S. Pat. No. 5,399,462, issued Mar. 21, 1995 to Sachev et al., entitled, “Method of Forming Sub-Half Micron Patterns With Optical Lithography Using Bilayer Resist Compositions Comprising A Photosensitive Polysilsesquioxane;” and U.S. patent application Publication 2001/0031420A1, published Oct. 18, 2001 to Lee et al., entitled “Partially Crosslinked Polymer For Bilayer Photoresist;” and U.S. patent application Publication 2001/0004510A1, published Jun. 21, 2001 to Wheeler., entitled “Refractory Bilayer Resist Materials For Lithography Using Highly Attenuated Radiation,” which are all incorporated by reference for all purposes discuss bi-layer photoresist methods that use an oxygen reactive ion etch (RIE) to etch an underlayer.
To facilitate understanding, FIG. 1 is a flow chart of a bi-layer photoresist process. First, an underlayer may be formed over a substrate (step 104). FIG. 2A shows a substrate, formed by a wafer 204 and a layer to be etched 208. An underlayer 212 may be formed over the layer to be etched 208. The layer to be etched 208 may be part of the wafer 204 or there may be one or more layers between the wafer 204 and the layer to be etched 208. The substrate may be the layer to be etched 208 or the wafer 204 or both.
A resist top image layer may be formed over the underlayer (step 108). FIG. 2B shows a resist top image layer 216 formed over the underlayer 212. The top image layer 216 may be exposed to patterned radiation (step 112). The pattern may then be developed in the top image layer (step 116). FIG. 2C shows a hole 218 that has been developed in the top image layer 216 as a result of the patterned radiation. The image may be transferred from the top image layer 216 to the underlayer 212 using an oxygen reactive ion etch (step 120). FIG. 2D shows a trench 220 that has been etched into the underlayer 212 as a result of the oxygen reactive ion etch. Oxygen RIE uses oxidation to etch, thus providing an oxidation dry etch.
Some of the top image layer 216 has been etched away during the transfer of the image, as shown. To increase the etch selectivity of the underlayer 212 to the top image layer 216 during the oxygen reactive ion etch, silicon is added to the top image layer 216. Even with silicon addition to the top image layer 216, selectivity may not be as high as desired, which may require the top image layer 216 to be thicker than desired or the underlayer 208 to be thinner than desired. In addition, the oxygen reactive ion etch may cause faceting 222 of the top image layer 216, which may enlarge the hole, increasing the critical dimension. In addition, the oxygen reactive ion etch may cause undercutting of the top image layer 216, which also increases the critical dimension. As a result of faceting and erosion, the critical dimension, of the hole at the top surface of the layer to be etched 208 may be much greater than the original trench size shown by dotted lines 226. To ensure that the underlayer is completely etched, the underlayer is normally over etched. The oxygen RIE may provide a large part of the CD enlargement during the over etch, with the least enlargement occurring at the beginning of the etch. In addition, the addition of silicon and the erosion of the top image layer 216 may cause silicon etch residue 230 to form on the surface of the layer to be etched 208, which may cause subsequent micromasking. The silicon may also form on a surface of the underlayer 212 during the etching of the underlayer 212, which may cause micromasking of the underlayer.
The image is transferred from the underlayer 212 to the substrate by an etch (step 124). FIG. 2E shows the layer to be etched 208 after a hole 234 has been etched. During the etch the top image layer is completely etched away, so that the underlayer acts as a pattern mask during the etch. The critical dimensions of the hole 234 are larger than the original trench size, shown by dotted lines 226. In addition, micromasking has formed bumps 238 at the bottom of the hole.
It would be desirable to provide better critical dimension control and reduce micromasking.